There are many situations wherein various possible sources of radiation exist in an area and these sources vary widely in frequency. Such situations usually require that a particular source of radiation be identified very quickly. A typical example involves a fighter plane. There are usually many possible radiation sources in the area where the pilot is flying. He must be able to quickly detect and recognize the radiation sources he is interested in such as a target. The pilot's equipment must have the capability of examining a large band width in a small amount of time and determine in a fraction of a second what it sees.
Broadband optical equipment is particularly well adapted to this situation. An infrared laser beam can be used effectively to detect radiation sources. However, a typical application of laser beams for this purpose utilizes an acousto-optic modulator to modulate the laser beam with an RF signal. The acousto-optic modulator is limited in band width and thus limits the use of the optical spectrum analyzer.